1. Technical Field
The present invention relates to an optical head driven by an area ratio gray-scale method, an exposure apparatus, and an image forming apparatus using the exposure apparatus.
2. Related Art
An image forming apparatus such as a printer includes an optical head used to form an electrostatic latent image on an image carrier such as a photoconductive drum. The optical head includes a plurality of light emitting devices arranged in the form of an array in a main scanning direction. Light emitting diodes may be used as the light emitting devices.
An example of a method of representing gradation is an area ratio gray-scale method (for example, see JP-A-2004-249549). An area ratio gray-scale method represents gray scales in units of blocks by expressing pixels belonging to blocks, each of which is constituted by n pixels in a main scanning direction and m pixels in a sub scanning direction, with binary values.
In the optical head using the light emitting diodes in the related art, a semiconductor chip and a driving IC are mounted on a print substrate with patterned wire thereon. It is important to realize basic print shading in an actual printing operation. In an optical head integrated with a driving circuit, since a head width is reduced and accordingly a circuit layout is extremely high in density, various signal lines have to intersect power lines of light emitting diodes. This produces parasitic capacitance at intersections of the signal lines and the power lines. Since the parasitic capacitance acts as coupling capacitance, when a signal of a signal line is changed from turn-on to turn-off and vice versa, noise is superimposed on a power line. As a result, current flowing in light emitting diodes turned on is varied, thereby temporarily varying luminance. Even a minute change in luminance may have an effect on the formation of an electrostatic latent image on a photoconductive drum, which may cause a user to recognize unevenness on a print sheet.
This problem becomes serious in a so-called “solid coating” print. This is because several pixel circuits make the same logic shift simultaneously, and accordingly, larger noise is added to a power line in the solid coating print. For example, a head of an A3 sheet 600 dpi requires about 8000 pixels. In this case, although 8000 pixels are divided into blocks, several hundred pixel circuits have to operate simultaneously, which may result in a very high level of noise.